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Evolution and character of complex strike-slip faulting in southern and offshore California

Mark R. Legg, & Marc J. Kamerling

Submitted September 10, 2023, SCEC Contribution #13118, 2023 SCEC Annual Meeting Poster #149

Complex earthquake ruptures as exemplified by Hector Mine, Landers; and Kaikoura NZ with combined deep thrust and shallow strike-slip faulting, demonstrate that simple models of strike-slip faults are inadequate for earthquake hazard assessment. Active strike-slip fault zones in the Borderland and southern California exemplify complex fault structure produced during plate boundary evolution. Progressive deformation during changing plate boundary configurations, from subduction to oblique-rifting and finally transform faulting, involve complex changing boundary conditions that may be understated by laboratory or software-based models. Initial conditions change from oblique breakaway along the coast of the subduction forearc and accretionary wedge due to microplate capture, followed by increasing strike-slip within the growing rift. Vertical-axis rotation of large crustal blocks due to indentations in the breakaway, and changing relative plate motion vectors modified tectonic character from transtension to transpression partitioned across the Borderland. Right-stepping echelon character of major right-slip faults in the Borderland and onshore southern California contradicts the classic Wrench Fault Tectonic model. Confinement by a growing oblique-rift limits the length of “new” transform faults and produces the right-stepping character. In contrast, the Newport-Inglewood fault zone is a left-stepping echelon right-slip fault system suggesting growth in the evolving Los Angeles basin sedimentary section, while at depth the fault is a major discontinuity between Catalina Schist and Peninsular Range basement rocks. With southward continuation as the Coastal Fault System, it stretches from the Santa Monica Mountains to Punta Banda, and farther along the Northwest Baja coast to Vizcaino Bay. Segmentation of faulting in the larger system exists at multiple scales depending upon pre-existing structure. The late Cretaceous strandline follows this system suggesting an ancient and profound crustal boundary. Changing rheology due to heating during oblique rifting, and crustal thickening due to convergence at the Transverse Ranges and San Andreas restraining bend, further complicate the regional tectonic evolution. Models derived to anticipate large earthquakes within this system must account for these complex boundary conditions that continue to change as the southern California transform plate boundary evolves.

Key Words
tectonic evolution, oblique-rift, strike-slip faulting, complex deformation

Legg, M. R., & Kamerling, M. J. (2023, 09). Evolution and character of complex strike-slip faulting in southern and offshore California. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)